Satellite images show improvements have continued at a North Korean nuclear facility, even though leader Kim Jong-un recently made commitments to dismantle the country's nuclear program.

Three researchers from 38 North, which closely follows North Korean affairs, said in a recent report that developments appear to be continuing "at a rapid pace" at the Yongbyon Nuclear Research Facility.

However, they said it is hard to determine whether some key pieces of infrastructure are operational.

On June 12, North Korean leader Kim Jong-Un met US President Donald Trump in Singapore, where they signed a broad agreement to denuclearise the Korean Peninsula.

Jenny Town, managing editor of 38 North, wrote on Twitter that the improvements seen at the nuclear site show why a concrete deal was needed instead of a "statement of lofty goals".

Changes the analysts noted in the photos taken on June 21 include completed modifications to the cooling system at one of the facility's reactors, newly erected buildings on the site and depleted coal bins.

The authors of the report caution that continued work at the nuclear facility is not necessarily related to North Korea's pledge to denuclearise.

"The North's nuclear cadre can be expected to proceed with business as usual until specific orders are issued from Pyongyang," the report said.

The images of the cooling system at Yongbyon's 5 MWe reactor show that improvements, which started in March, now appear to be complete.

When signs of a major excavation appeared in earlier satellite images of the site, the analysts at 38 North concluded that improvements were being made to the reactor's water cooling system so that the facility could operate more continuously and more safely.

The most current images now indicate a full water channel and possibly a weir to control water flow, as well as a new pump house.

Although the equipment appears to be discharging a smaller amount of water than previously, the report said it is hard to conclude from this whether the reactor is fully operational or not.

"It may simply be that this is all the water needed at this time to carry away the heat from the residual radioactivity in the reactor, if the reactor is currently shut down," the report said.

According to 38 North, there is no evidence to suggest that operations have begun at this facility, but from the satellite images it appears that the necessary infrastructure for the reactor to operate is now complete.

A new small building has been completed at the site. It is of similar construction to one at the radiochemical laboratory, and does not appear to be industrial in design, so could serve a hospitality function for senior officials.

Water is now visible in a sluiceway that diverts part of the Kuryong River for use in the cooling systems of the Yongbyon reactors, and the reservoir that provides the water appears to be at the level it was designed for.

There is no visible vapor coming from the cooling towers at this facility, so the 38 North report said it is hard to tell whether the plant is operational.

"There is no visible evidence yet to suggest that operations have begun," the report said.

However, the movement of vehicles around the site, which separates plutonium from spent fuel from the 5 MWe reactor, does suggest it is active.

The new images show a small, non-industrial building of similar design to the one at the light water reactor has been completed.

Three large coal bins at the thermal plant appear to have been depleted, but the authors of the report caution there is no visible smoke rising from the smoke stack at this site, so it is hard to conclude whether it is in operation.

The analysts believe the images show operations at the uranium enrichment plant are ongoing.

They said discoloration of the roofs of the cascade halls, which contain the plant's centrifuges, is caused by ice forming from water vapor emitted by the facility's cooling units.

Based on earlier images, this facility is thought to be a radioisotope production facility.

An underground pipeline now appears to have been laid between the main production building and two buildings completed earlier this year that could store chemicals arriving at a nearby rail yard.